Suzanne Simard: How Do Trees Collaborate? Ecologist Suzanne Simard shares how she discovered that trees use underground fungi networks to communicate and share resources, uprooting the idea that nature constantly competes for survival.
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How Do Trees Collaborate?

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How Do Trees Collaborate?

How Do Trees Collaborate?

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It's the TED Radio Hour from NPR. I'm Guy Raz. And on today's show ideas about the Power Of Networks, how those connections and those pathways define the world around us in our cities, in our relationships, in our bodies and especially in nature.

So about 25 years ago, forest ecologist Suzanne Simard had a hunch.

SUZANNE SIMARD: Yes, that's right.

RAZ: She thought that trees could talk.

SIMARD: Just imagine, like, when you're walking through the forest you might - you hear the crunching of the twigs under your feet and the rustling of the leaves.


RAZ: But she thought - what if there's more going on?

SIMARD: Like a big chattering going on that we can't hear, that they're attuned to each other.

RAZ: Now, at the time - and again, this is about 25 years ago - a team of scientists in England were wrapping up an experiment.

SIMARD: Where they'd grown, in the laboratory, these pine seedlings together in little root boxes that you could see through.

RAZ: And the scientists took two of these pine seedlings, these baby trees, that were in the same box, in the same dirt and then they exposed one of these seedlings to a radioactive carbon dioxide gas.

SIMARD: Carbon-14, a radioactive carbon.

RAZ: And what they found was that some of that radioactive gas, the carbon-14, made its way into the second seedling.

SIMARD: You could visualize it. You could see it.

RAZ: And so from this experiment it seemed that somehow these two plants, in the same dirt, were connected.

SIMARD: And I thought, wow, you know, maybe this is what's going on in my forest.

RAZ: Maybe, Suzanne Simard thought, maybe all the trees in a forest were connected in a kind of network.

SIMARD: You know, like our airport system, our transportation system, our social networks.

RAZ: And maybe, she thought, all of this was happening underground.

SIMARD: When we walk through the forests what we see as human beings - we just see these, you know, beautiful trees growing out of the ground. But we don't see that they're actually completely linked underground in this super-highway.

RAZ: So Suzanne decided to prove this underground network existed. She devised an experiment using some of the same radioactive gas, a Geiger counter to measure it and a patch of birch and fir trees.


SIMARD: I figured the birch and the fir would be connected in the below-ground web.

RAZ: Suzanne picks up the story from the TED stage.


SIMARD: And I gathered my apparatus, plastic bags, and duct tape, and shade cloth, a paper suit, a respirator. And then, I borrowed some high-tech stuff from my university - a Geiger counter, a mass spectrometer, microscopes. The first day of the experiment we got out to our plot, and I pulled on my white paper suit. I put on my respirator. I put the plastic bags over my trees. I got my giant syringes, and I injected carbon-14, the radioactive gas, into the bag of birch.

I waited an hour. I figured it would take this long for the trees to suck up the CO2 through photosynthesis, send it down into their roots and maybe shuttle that carbon below ground to their neighbors. I went to my first bag with the birch. I pulled the bag off. I ran my Geiger counter over its leaves, (imitating Geiger counter). Perfect, the birch had taken up the radioactive gas. Then, the moment of truth. I went over to the fir tree. I pulled off its bag. I ran the Geiger counter up its needles. And I heard the most beautiful sound, (imitating Geiger counter).


SIMARD: It was the sound of birch talking to fir. And birch was saying, hey, can I help you? And fir was saying, yeah, can you send me some of your carbon? I was so excited.


SIMARD: I ran from plot to plot. And I checked all 80 replicates. The evidence was clear. Paper birch and Douglas fir were in a lively two-way conversation.

It turns out at that time of the year, in the summer, that birch was sending more carbon to fir than fir was sending back to birch, especially when the fir was shaded. And then, in later experiments, we found the opposite, that fir was sending more carbon to birch than birch was sending to fir. And this was because the fir was still growing while the birch was leafless. So it turns out the two species were interdependent, like yin and yang.

And at that moment, everything came into focus for me. I knew I'd found something big, something that would change the way we look at how trees interact in forest, from not just competitors but to cooperators.

RAZ: Now, you have to understand that Suzanne's discovery was pretty revolutionary because up until this point, most ecologists believe that trees competed against each other, that their world was, like, a Darwinian struggle with winners and losers.

SIMARD: Yeah, you know, that they're competing for light and water and nutrients.

RAZ: And that the strongest trees were the ones that grew tall, the ones that dominated the canopy and took all the resources.

SIMARD: It was like, oh, I'm going to get what I want. And I don't care what my neighbor needs.

RAZ: But Suzanne's experiment showed that something else was true.

SIMARD: They're actually sending messages back and forth that balances the resource distribution among the community.

RAZ: In other words, trees aren't just connected. They're actually sharing resources with each other.

SIMARD: So what we found initially, if one tree had a lot of water in it or a lot of nitrogen or had high photosynthetic rate and if one tree is sick, then the neighboring tree shuttles more of those nutrients to that suffering tree.

RAZ: And when you say communicate do they actually communicate? Like, do they warn each other about like a fire or an invasive species or something?

SIMARD: Yes, so if one tree gets damaged by say mountain pine beetle, the injured seedling will up its defense enzymes. And then, the receiving tree will then increase its defense enzymes because it knows now that there's some kind of damaging agent around.

RAZ: Wow, so how are they doing this? Like, how are they communicating through an underground network?

SIMARD: So they're physically connected by these microscopic fungi and...

RAZ: So mushrooms?

SIMARD: Yes, you're right. We call them hyphae or mycelium. In fact, like, if you were to, you know, peel back the surface of the forest floor you'll see the fungi that are linking these trees together. They're very visible. And it's these white and yellow, different colored threads that are - they look like, you know, sewing threads. But they're fungal threads. And they're crisscrossed and going off in multiple directions. And they work together to create a very, you know, a very complex web. And they're in constant communication between all the trees.

RAZ: So this network is called the mycorrhizal network. And Suzanne wanted to see how intricate it actually was. So she built a map - a massive interconnected map - where each tree represents a circle or a node.


SIMARD: The biggest, darkest nodes - we call those hub trees, or more fondly, mother trees, because it turns out that those hub trees nurture their young, the ones growing in the understory. And if you can see those yellow dots, those are the young seedlings that have established within the network of the old mother trees. In a single forest, a mother tree can be connected to hundreds of other trees. We have found that mother trees will send their excess carbon through the mycorrhizal network to the understory seedlings. And we've associated this with increased seedling survival by four times.

Now, we know we all favor our own children. And I wondered - could Douglas fir recognize its own kin? So we set about an experiment. And we grew mother trees with kin and strangers seedlings. And it turns out they do recognize their kin. Mother trees colonize their kin with bigger mycorrhizal networks. They send them more carbon below ground. They even reduce their own root competition to make elbow room for their kids. When mother trees are injured or dying they also send messages - wisdom onto the next generation of seedlings. And these have increased the resistance of those seedlings to future stresses.

RAZ: It's almost like with these trees, and especially what the mother trees are doing - it's almost like a selfless act.

SIMARD: Well, it seems like that at the surface. But when you start digging down into the multiple interactions going on in the forests, it's not really a selfless act because trees need a complex or a diverse community to thrive in. They need, you know, other plants that can cycle nutrients more quickly or that can access nutrients in different niches. They need other neighbors that are resistant to insects and diseases. So it's actually in their self-interest to be sharing these resources with their neighbors, to make sure their neighbors, their diverse community is vibrant because that feeds back to them. And then, they're more vibrant and healthy.

RAZ: Yeah, it's almost like they need each other.

SIMARD: Yes, I think, you know, one way for us to think about this, you know, we live in communities of, you know, doctors and teachers and people that are running the coffee shops and bakers. And there's a whole range of skills that make a thriving community. We need each other. If you take away, you know, the baker then we've got no bread. If we take away the banker, where do we get our money?

SIMARD: So it's the same in a forest. There's all these different species, and they're all part of this holistic functioning ecosystem. They all have a role to play just like in our human communities. And if you lose one of those key individuals, then the whole thing has to reorganize itself to make up for what's missing.


SIMARD: Forests aren't simply collections of trees. They're complex systems with hubs and networks that overlap and connect trees and allow them to communicate, and they provide avenues for feedbacks and adaptation. And this makes the forest resilient. That's because there are many hub trees and many overlapping networks, but they're also vulnerable because hub trees are not unlike rivets in an airplane. You can take out one or two and the plane still flies. But you take out one too many or maybe that one holding on the wings, and the whole system collapses. Well, you know, the great thing about forests as complex systems is they have enormous capacity to self-heal.

In our recent experiments, we found with retention of hub trees every generation to have diversity of species and genes and genotypes that these mycorrhizal networks - they recover really rapidly. We need to regenerate our forests with a diversity of species and genotypes and structures by planting and allowing natural regeneration. We have to give Mother Nature the tools she needs to use her intelligence to self heal, and we need to remember that forests aren't just a bunch of trees competing with each other. They're super cooperators. Thank you.


RAZ: Forest ecologist Suzanne Simard. She's based at the University of British Columbia. You can see her entire talk at Our show today - ideas about the Power Of Networks. Stay with us. I'm Guy Raz, and you're listening to the TED Radio Hour from NPR.

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